The present invention relates to a centrifugal compressor having an oil film seal system and a shaft seal therefor, and in particular, relates to a multi-stage centrifugal compressor having an oil film seal system and a shaft seal system therefor.
Heretofore, there has been widely used so-called oil film seals each having a seal ring fitted on a rotary shaft with a clearance therebetween, and oil is fed to the outer peripheral surface of the sealing ring through an oil feed passage formed in the casing side in order to seal working gas and bearing lubrication oil in a centrifugal compressor. For example, Japanese Laid-Open Patent No. 08-121109 discloses a device using an oil film seal, as mentioned above.
The oil film seal has the following features:
(a) an oil film formed in the oil film seal system can completely seal working gas in a centrifugal compressor so as to prevent it from leaking from the compressor.
(b) This oil film seal is a noncontact type seal so that no sliding parts including a mechanical seal or a lip seal are incorporated, and accordingly, friction and abrasion are less so that the long term continuous operation can be made. Further, due to the noncontact type, it can be used even though the peripheral speed of the rotary shaft of a seal part is high (about 80 m/s), and accordingly, it can be used in a high power machine.
(c) By changing the pressure of oil to be fed, the working gas in the compressor can be sealed so as to be prevented from leaking into the outside of the compressor even though the pressure of the gas is high.
(d) The oil film seal can remove heat generated in the seal part through the seal oil, and accordingly, the seal has highly safe and reliable.
The oil film seal forms an oil film having a pressure which is slightly higher than a pressure of the working gas in the compressor, between the sealing ring and the shaft, so as to seal the gas. Further, a single oil film seal system has two seal rings one of which is located on the inside side (gas side) of a centrifugal compressor and the other one of which is located on the outside (atmospheric side) of the compressor. These seal rings are incorporated in a seal housing formed in a casing so as to define extremely narrow clearances between themselves and a rotary shaft and to follow fine motion of the rotary shaft. Oil having a pressure which is slightly higher than the gas in the compressor is fed into a space between two seal rings, one of which is on the inside of the compressor and the other is on the outside of the compressor. Some of the fed oil flows toward the atmospheric side seal ring while the remainder thereof flows into the compressor side through the annular clearance between the seal ring on the inside of the compressor and the shaft by a slight amount. Thus, it is possible to prevent the working gas in the compressor from flowing into the atmospheric side.
In this conventional oil film seal system, oil discharged from the gas side is made into contact with gas in the compressor. Accordingly, the seal oil is excessively contaminated by the gas in the compressor if the gas is of a certain kind. Discharged oil which is possibly contaminated by a large degree, is led into a drain trap where the gas is separated from the oil, and then, the oil is retrieved or exhausted. In order to minimize the quantity of the exhausted oil which is so-called xe2x80x9csour drainxe2x80x9d, the following methods have been conventionally used:
1) The differential pressure between the supplied oil and the gas inside of compressor to be fed is lowered; and
2) The clearance between the gas side seal ring and the rotary shaft is reduced.
If the differential pressure in the compressor is extremely small as stated in item 1, the gas is likely to leak into the outside from the compressor, and meanwhile, if the clearance is extremely small as stated in item 2, the temperature at the inner surface of the gas side seal ring is raised excessively, resulting in that the seal ring is damaged by burning, or the possibility of a risk of making the seal ring into contact with the rotary shaft is increased. Indeed, there have been reported many accidents such that the gas side seal ring is damaged by burning. In particular, should any trouble occur in a centrifugal compressor as one of important components of any of various plants including a petroleum refining plant and a chemical plant, such a plant would be inoperative throughout the system. Thus, it is very important to ensure the reliability for the shaft seal system including the oil film seal system.
Further, the seal ring is fitted on the rotary shaft with a clearance defined therebetween, and the so-called sealing sleeve is normally shrinkage-fitted on the rotary shaft in a part facing the outer seal ring in order to protect the rotary shaft surface. Since seal oil is fed at the outer peripheral side in the oil film seal, this seal ring is heated by the seal oil so as to expand, and accordingly, unbalance load would possibly be applied to the rotary shaft. As a result, there would have been such a disadvantage that the rotor including the rotary shaft incurs unbalance vibration.
The present invention is devised in view of the above-mentioned problems and disadvantages inherent to the prior art, and one object of the present invention is to provide an oil film seal used in a centrifugal compressor with an enhanced reliability. Further, in particular, an object of the present invention is to provide an oil film seal system which can prevent a gas side seal ring from being damaged by burning and consequently enhance the reliability.
Another object of the present invention is to provide a centrifugal compressor incorporating an oil film seal system which can reduce unbalance vibration of a rotor caused by unbalance of a seal part.
The other object of the present invention is to provide a centrifugal compressor incorporating an oil film seal system which can achieve the above-mentioned objects with only a simple structure.
To the end according to the first aspect of the present invention, there is provided a centrifugal compressor comprising a rotary shaft, a casing attached thereto with bearing means for rotatably journalling the rotary shaft, at least one centrifugal impeller fitted to the rotary shaft, a shaft seal system for preventing gas compressed by the centrifugal impeller from leaking through a gap between said rotary shaft and the casing, the shaft seal system comprising a gas side seal ring arranged axially outside of the impeller with a radial clearance between itself and the casing, and an atmospheric side seal ring located outside from the gas side seal ring with a radial clearance between the casing, and an oil feed passage for supplying oil to both seal rings is formed in the rotary shaft.
Preferably, the bearing means are provided at two positions in the axial direction, and two shaft seal systems are provided inward from the bearing means in the axial direction of the compressor, or alternatively, the oil passage formed in the rotary shaft has an opening at one end of the rotary shaft, an oil feed bore passing through the center axial part of the rotary shaft, a plurality of oil feed holes communicated with the oil feed bore and formed in the rotary shaft so as to be extended radially outward, the plurality of feed holes being located at an axial position which is in an axially intermediate part of the gas side seal ring, the rotary shaft is incorporated thereon with sealing sleeves for covering the rotary shaft, corresponding to positions where the gas side seal ring and the atmospheric side seal ring are provided respectively, the rotary shaft is provided with a chamber for reserving oil fed from the oil feed bore, at the one end side where the oil feed bore is opened, and mechanical seal means for sealing between the rotary shaft and the casing is provided axially inward from the chamber.
In order to achieve the above-mentioned objects, according to a second aspect of the present invention, there is provided a centrifugal compressor having oil film seal systems provided at two positions in the axial direction of the rotary shaft, a first bore is formed in the center axial part of the rotary shaft, and second holes are formed in the rotary shaft at positions where the oil seal systems are attached, being communicated with the first bore and opened at the outer periphery of the rotary shaft.
Preferably, there are provided a head tank for reserving oil adapted to be fed into the oil seal systems, and a control means for controlling the level of the oil in the head tank. Alternatively, the oil film seal system has a gas side seal ring and an atmospheric side seal ring, a degassing means for cooling the gas side seal ring so as to remove working gas components from oil mixed therewith, and a reservoir for mixing oil with which the atmospheric side seal ring has been cooled, and the oil from which the working gas is removed by the degassing means, with each other.
In order to achieve the above-mentioned objects, according a third aspect of the present invention, there is provided an shaft seal system for a centrifugal compressor, comprising a gas side seal ring and an atmospheric side seal ring which are loosely fitted in a housing, and sealing sleeves located on the inner diameter sides of these seal rings, and each having a plurality of holes radially extending through the sleeves.
Preferably, there are provided a head tank for reserving oil adapted to be fed to both seal rings through he plurality of holes at the inner diameter side, and a control means for controlling the level of the oil in the head tank.